An electrochromic device is a self-contained, two-electrode (or more) electrolytic cell that includes an electrolyte and one or more electrochromic materials. Electrochromic materials can be organic or inorganic, and reversibly change visible color when oxidized or reduced in response to an applied electrical potential. Electrochromic devices are therefore constructed so as to modulate incident electromagnetic radiation via transmission, absorption, or reflection of the light upon the application of an electric field across the electrodes. The electrodes and electrochromic materials used in the devices are dependent on the type of device, i.e., absorptive/transmissive or absorptive/reflective.
Absorptive/transmissive electrochromic devices typically operate by reversibly switching the electrochromic materials between colored and bleached (colorless) states. Typical electrochromic materials used in these devices include indium-doped tin oxide (ITO), fluorine-doped tin oxide (SnO2:F), poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS), and single-walled carbon nanotubes (SWNT). An exemplary electrochromic device of this type has been constructed using a substrate layer of polyethylene terephthalate (PET), a transparent layer of ITO as the working electrode, and a third layer of PEDOT-PSS as the counter electrode.
The absorptive/reflective-type electrochromic devices typically contain a reflective metal as an electrode. The electrochromic material is deposited onto this electrode and is faced outward to allow incident light to reflect off the electrochromic material/electrode surface. The counter electrode is behind the active electrode. Similar electrode and electrochromic materials can be used in these reflective devices, in particular ITO and PEDOT-PSS.
Traditionally built electrochromic devices utilizing an electrochromic polymer have a discrete electrochromic polymer layer assembled with an electrolyte on top. Devices are assembled between two electrodes using the electrolyte between them to achieve the necessary ion shuttling for the redox-active electrochromic polymers. This electrolyte is often cross-linked into a gel.
In traditional processes to prepare the foregoing electrochromic devices using an electrochromic polymer such as PEDOT, the electrochromic polymer is formed into a discrete thin film prior to device assembly. Typical processes to prepare the thin film are via electrodeposition, spin or spray casting from solutions, etc. Drawbacks to using electrodeposition include the use of costly and wasteful electrolyte baths, the need for the frequent changing of organic salts and solvents in the baths, as well as the need for proper disposal of spent baths. Electrodeposition processes are also known to have poor yields.
Other processes besides electrodeposition involve complex syntheses to generate soluble versions of an electrochromic polymer which can then be cast and assembled into a device. The use of so-called precursor polymers can be used in a casting process and then converted to their electrochromic counterpart. However, such a process still involved the initial preparation of an electrochromic polymer film prior to device assembly.
There remains a need in the art for processes to prepare electrochromic devices. There also remains a need for electrochromic devices having improved properties.